i can't say whether it was placed there or part of the environment. >> reporter: the shrapnel is just one of the clues. >> there are fragments in the victimance in the salk and what they will be able to do is probably reconstruct the timer device, what was used and look it will tell them if someone put it together over the internet or is this a bomb maker. >> reporter: hundreds of state and federal investigators have descended on boston to figure out who built the bombs and set them off. >> federal and state and local law enforcement are coordinating closely. the fbi has taken charge of the investigation. >> reporter: this type of attack has been law enforcement's worst fear. a soft target where suspects can blend in and inflict mass casualties. >> unfortunately these kinds of settings, a marathon, a concert, they are attractive targets. >> reporter: the boston marathon is a heavily policed event. more than 500 national guard troops were on scene this afternoon as well as members of the boston police and fire department. but that did not prevent this from happening. >> anyone can walk